1. Field of the Invention
The present invention relates to a projection electronic microscope for irradiating a surface of a sample with electron beams to make observation, testing and the like of the surface of the sample using resulting secondary electrons, reflected electrons and the like, and more particularly, to a projection electronic microscope for reducing geometric aberration and space charge effect.
2. Description of the Related Art
A projection electronic microscope irradiates a wide area on a surface of a sample with electron beams emitted from an irradiation electro-optical system (primary electro-optical system), collectively focuses secondary electrons and reflected electrons emitted from the surface of the sample on a detection plane of a detection system using an image formation electro-optical system (secondary electro-optical system) for two-dimensionally observing the surface of the sample. Such a projection electronic microscope draws attention for use as an apparatus for testing micro-devices such as semiconductors, because of its abilities to observe a wider area on a surface of a sample with a smaller number of scans than a conventional SEM, and therefore reduce a sample observation time (see JP-2004-209429A).
FIG. 1 illustrates an exemplary configuration of a conventional projection electronic microscope. The projection electronic microscope 20 comprises an electron gun 21 for emitting a primary electron beam for irradiating a sample W therewith; an illumination optical system 22 for reshaping the primary electron beam and guiding the reshaped primary electron beam to the sample W; a beam separator 23 for changing a direction in which the primary electron beam travels to be perpendicular to the sample W, and separating a secondary electron beam such as secondary electrons, reflected electrons and the like emitted from the sample with the irradiation of the primary electron beam from the primary electron beam; a projection/image formation optical system 24 for reshaping the secondary electron beam; and a detection system 25 for two-dimensionally detecting the secondary electron beam. The projection/image formation optical system 24 for guiding the secondary electron beam to the detection system 25 comprises an objective lens system 24a, a transfer lens system 24b, and projection lens system 24c which are arranged along a linear optical axis 26 of the secondary electron beam.
Describing the operation of the projection electronic microscope illustrated in FIG. 5 in brief, the electron beam 21 generates a primary electron beam. The generated primary electron beam is incident on the illumination optical system 22, and is reshaped by the illumination optical system 22 into a beam which fits in a required irradiation range. The reshaped primary electron beam is incident on the beam separator 23 which deflects the motion direction of the primary electron beam in a direction perpendicular to the surface of the sample W. Next, the primary electron beam is incident on the objective lens system 24a of the projection/image formation optical system 24, is reshaped by the objective lens 24a into an electron beam which has a uniform irradiation intensity on the surface of the sample W, and is irradiated perpendicularly to the surface of the sample W.
From the surface of the sample W irradiated with the primary electron beam, a secondary electron beam is emitted in the direction perpendicular to the surface of the sample W. The secondary electron beam has a distribution in accordance with the shape, a material distribution, a change in potential, and the like of the surface. The objective leas system 24 converges the secondary electron beam to focus the same on a center plane of the beam separator 23. The focused secondary electron beam travels along the optical axis 26 without being deflected by the beam separator 23, and is focused in front of the projection lens 24c by the transfer lens system 24b. Further, the thus focused secondary electron beam is focused on a detection plane 9 of the detection system 25 by the projection lens system 24.
FIG. 2 illustrates an exemplary configuration of the projection/image formation optical system 24 in detail, where the transfer lens 24b is used as a zoom type projection/image formation optical system which additionally serves as a zoom system. In FIG. 2, the objective lens group 24a comprises a first objective lens 4a, a second objective lens 4b, and a diagram 5 disposed therebetween. The transfer lens system 24b comprises a first zoom lens 6a and a second zoom lens 6b. The projection lens system 24c comprises a first projection lens 8a and a second projection lens 8b. 
In FIG. 2, secondary electron beams 3 are emitted from a sample surface 1 in a direction along the optical axis 26 with the irradiation of the primary electron beam. In FIG. 6, a peripheral electron beam 3a refers to a beam emitted from an axial point on the sample surface 1, and main electron beam 3b refers to a beam emitted from an off-axis point on the sample surface 1 among the emitted secondary electron beam 3. Motions of these electron beams will be described below in brief.
The peripheral electron beam 3a emitted from the sample surface 1 is collimated by the first projection lens 4a to be in parallel with the optical axis 26, passes through the diagram 5, is converged by the second objective lens 4b to focus on the center plane 7 of the beam separator 23, and is then converged by the first zoom lens 6a and second zoom lens 6b to focus near the center point of the first projection lens 8a. The thus focused peripheral electron beam 3a is converged by two projection lenses 8a, 8b of the projection lens system 24c to focus on the detection plane 9 of the detection system 25.
On the other hand, the main electron beam 3b is incident on the first objective lens 4a, and then produces a first cross-over C1 at the center of the diagram 5. The main electron beam 4b which has passed through the diagram 5 is collimated by the second objective lens 4b to be in parallel with the optical axis 26, is incident on the first zoom lens 6a, and produces a second cross-over C2 in front of the second zoom lens 6b. Subsequently, the main electron beam 3b is converged by the second zoom lens 6b and two projection lenses 8a, 8b, and forms a third cross-over C3 near the center point of the second projection lens 8b. 
Incidentally, it is known that the resolution of a projection electronic microscope is mainly determined by geometric aberration of the projection/image formation optical system 24 and the space charge effect of the secondary electron beams. This is because the projection/image formation optical system 24 employs electrons which have low kinetic energy of several eV on the sample side and several KeV in a field free space between the lens systems. For example, in the zoom type projection/image formation optical system as illustrated in FIG. 2, the secondary electron beams 3 are focused three times in order to achieve a highly magnified image formation (image formation when the projection/image formation optical system is set at a high magnification). Accordingly, the secondary electron beams converge three times, i.e., produce three cross-overs C1-C3. Since the electron density is extremely high in regions in which such cross-overs are produced, as compared with other regions, the space charge effect becomes larger. In other words, an increase in the number of cross-overs causes the space charge effect to significantly increase.
Also, in the zoom type projection/image formation optical system 24 as illustrated in FIG. 2, the size of an observation field varies in accordance with the magnification with respect to the same detection area. Therefore, when the transfer lens system 24b is required to provide a larger zoom range than an ordinary zoom range of one to three times, the viewing field is wider in a lowly magnified image formation (image formation when the projection/image formation optical system is set at a low magnification), as compared with a highly magnified image formation, with a larger angle over which the beams diverge, resulting in another problem of a significant deterioration of off-axis geometric aberration.